Searching for oil or more generally hydrocarbons has become more demanding in terms of hardware and devices in recent years because oil and gas fields or reservoirs are located deeper or in places difficult to reach and below the sea floor. Prospecting for and exploitation of hydrocarbon fields make it necessary to use hardware which is more resistant to environmental challenges such as higher loads and corrosion, which were less important in the past.
Modern joints are generally designed with metal to metal seals made by the contact between two surfaces, usually at one end or at both ends of the threaded portion of the joint, interfering in the elastic range of the modulus of elasticity for an appropriate stress magnitude. However in specific situations, resilient seals are needed instead of or in combination with metallic seals, to prevent penetration of external fluids in the interstices of the threads.
It is therefore a design requirement that the joint seals resist penetration of external or internal fluids, or at least do not allow continuous exchange of fluids that have already penetrated the joint with the surrounding fluids, in order to reduce the corrosion rate.
To solve the mentioned problems various solutions have been provided, one of which consists of inserting a seal ring made of plastic material, e.g. an O-ring, at the extremity of the joint external to the threaded portion.
It is a widespread practice to use O-rings pre-mounted onto the box, but this practice has two disadvantages: the seal rings can be damaged during mounting of the ring and/or during make-up, and O-rings of simple cross-section cannot offer redundant sealing functions.
Seal rings with more sophisticated shapes have also been proposed to increase efficiency and prevent external fluids from penetrating into the gaps of the joint. However such seal rings are subject to large stresses and adverse environmental conditions, both during the initial make-up operation and during operational life, such that their efficiency is much reduced after some time.
Another drawback of such elastic seal rings is the damage caused during the make-up operation because operators in the field can cause shocks between the tubes. The seal ring can thus be torn or suffer permanent deformation which later can jeopardize the sealing efficiency during operational life.
Document WO2004053376 discloses a threaded connection for connecting pipes for hydrocarbon wells with tapered male and female threads; an annular housing is provided in the vicinity of the free end of the female element, which housing receives a portion of a deformable sealing ring, which is radially compressed, while a retaining lip of the ring is immobilized in a further annular housing of the female element by a rib on the male element. However such a lip is a weak and fragile element due to its small cross-section, and its shape requires that the sealing ring be manufactured with a material having high slidability. It may easily break leaving the seal ring unretained and free to move axially. Moreover the seal ring is made of a material with low friction coefficient, reducing the available materials that can be used.
Such a seal ring can be considered as a seal having multiple sealing mechanisms working as a whole. Another disadvantage of this solution is that there is a need to machine customized annular ribs and/or grooves on the pin and box to position the seal ring which entails expensive and lengthy operations. Making the ribs and/or grooves requires high precision machining to provide the correct relative axial position of the rib or groove with respect to the nose of the box after make up. Moreover the seal ring is prone to twisting during make up, which makes operation difficult.